Synthesis and evaluation of a moisture-promoted healing copolymer
Graphical abstract
Introduction
As we know, skin plays a role of protection for the body in the biological systems, and it can also be self healed and recovered its protection ability again when it suffers damage. Inspired by this biological repairing system, coating material with self healing ability may perfectly protect the substrates and itself simultaneously [1], [2], [3], [4], [5], which attracted more and more interests of researchers [6], [7].
As the mainstream of self healing systems, the extrinsic healing system which embedded liquid healing agent into microcapsules [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], hollow fibers [24], [25], microvascular [26], core-sheath structure [27] and so on [28], [29], [30], [31], [32] was intensively focused on all the time. The embedded active healing agent would be released to the crack once mechanical damage occurred [33]. Afterwards, the healing agent can polymerize when contacting with the catalyst or hardener, which either dispersed in the matrix or released from other ruptured containers, resulting in rebinding of the crack faces. This approach, despite the sophistication in chemistry and manufacturing, has been quite successful in achieving high mending efficiencies and is applicable to various types of polymeric materials. Aside from the healing agent embedded system, the intrinsic self healing system can be achieved by physical interaction [34], reversible chemistry, which can recover itself by the formation of new covalent bonding [35], [36], [37], [38] or weaker interactions, such as hydrogen bonding [39], [40], [41], [42], [43], [44], π–π interactions [45], [46] polyelectrolyte multi layers [47], [48] and so on. For example, by incorporated an active precursor into the polymeric network, material could be well healed by the reaction of the active function groups on the both sides of the crack under the sun exposure after mechanical damage [49]. Because there was no heterogeneous “containers” in the polymeric network, the mechanical property could not be influenced; moreover, the healing trigger was UV light existed in the open air, which provided an ambient temperature approach to self healing.
For the metal anti-corrosion self healing coating materials the material structure designs are mainly focus on the systems with encapsulated reactive components in polymeric matrices. For example, Garcia et al. [50] synthesized a water-reactive healing agent based on a silyl ester and encapsulated it into an organic coating. The silyl ester could locally released at the damage site, wets both the metal and polymer surface, reacts with moisture which protects the metal at the scribe from further corrosion attack; Samadzadeh et al. [51] encapsulated Tung oil as an oxidative healing agent the epoxy coating; and Huang et al. [52] reported a facile microencapsulation preparation approach for liquid isocyanates hexamethylene diisocyanate monomer via an interfacial polymerization method in oil-in-water emulsion, which makes the instant healing possible.
Herein, we report a novel self healing coating system based on only one designed copolymer, which can heal itself under moisture environment. This copolymer consists of two components, isocyanate groups containing monomer (ITEGMA) and fluorinated monomer, 2, 2, 3, 4, 4, 4-hexafluorobutyl methacrylate (HFBMA). In this system, the fluorine-containing segments in HFBMA component, due to their low surface free energy, are prone to gather on the solid–air interface, to form a good water barrier during the coating preparation and application process [53], [54], [55]. And the isocyanate containing segments in ITEGMA component, due to their strong polarity, are prone to locate on the solid–substrate interface, to form a reparable layer. Once the coating is broken, isocyanate groups exposed can react with H2O in air and form a urea crosslink. The reactions are depicted in Equations (1), (2), making the moisture self healing process possible.
In this work, first, an isocyanate containing metharcylate monomer ITEGMA was synthesized according to the scheme as shown in Fig. 1. Then various compositions of poly (HFBMA-co-ITEGMA) copolymers were synthesized, and then the characteristics and the self healing property of the copolymers as metal protection coating were investigated. Finally, we speculate as the possible healing mechanism for this kind of self healing behavior.
Section snippets
Materials
Isophorone diisocyanate (IPDI, Degussa Co. LTD., German) was used as received. 2-Hydroxyethyl methacrylate (HEMA, Beijing Chemical Works, China) and 2, 2, 3, 4, 4, 4-hexafluorobutyl methacrylate (HFBMA, Xeogia Fluorine-Silicon Chemical, Harbin, China) were purified by vacuum distillation before use. 2, 2′-azobis (isobutyronitrile) (AIBN, Tianjin Fuchen Chemical, China) was recrystallized from ethanol. Triethylene glycol (TEG, Tianjin No.1 Chemical Reagent Factory, China) was dried under vacuum
Synthesis of isocyanate containing monomer
In this work, first, an isocyanate containing metharcylate monomer ITEGMA was synthesized according to the scheme as shown in Fig. 1, which is confirmed by infrared (IR) (Fig. 2) and electrospray ionization spectrometer (ESI-MS). From the ESI-MS spectrum of BITEG, the molecular ion peak is 617 (M + Na+ = 617). The measured molecular mass is in good agreement with calculated molecular weight of BITEG (M = 594). The molecular ion peak, 742 (M + NH4+ = 742), also matches the calculated ITEGMA
Conclusion
In this work, a novel poly (HFBMA-co-ITEGMA) copolymer composed of fluorine segments and isocyanate groups containing components has been synthesized. The self healing property was demonstrated when using the copolymer as a protect coatings on aluminum sheet. When the fluorine segment containing monomer is over 30%, the coating film of the copolymer can be protected well against the environment moisture. The optimized ratio of the self healing coating film was found out to be
Acknowledgments
This work was supported by National Natural Science Foundation of China (Proj. No. 50978138) and State Key Lab for Modification of Chemical Fibers and Polymer Materials, Donghua University, China (Proj. No. LK 0808) for financial support.
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